1. Technical Field of the Invention
The invention relates generally to communication systems; and, more particularly, it relates to communication receivers employing Code Division Multiple Access (CDMA).
2. Description of Related Art
Data communication systems have been under continual development for many years. There is oftentimes difficulty in accommodating the offset of operational frequencies between various devices within the communication system. Carrier frequency offsets (CFOs) are very common in data communications systems. When there is a frequency offset between a transmitter and a receiver within a communication system, there may be significant degradation in performance. The undesirable degradation in performance may result in an inability to demodulate and decode received data.
In order to correct for CFOs, an accurate estimate of the CFO is needed. In packet systems, a known preamble sequence is transmitted with each data packet. The received sequence and the known preamble are usually used at the receiver to estimate the CFO. The prior art includes a number of various methods and approaches that may be used to achieve a CFO. However, these prior art methods and approaches typically involve a high degree of computational complexity. In addition, these prior art methods and approaches often fail to provide for a high degree of accuracy. As such, accurate CFOs may typically not be achieved in most prior art communication systems. This results in a failure to achieve optimal performance and accurate system operation.
One particular type of communication system, a cable modem (CM) communication system, has been under continual development for the last several years. There has been development to try to provide for improvements in the manner in which communications between the CM users and a cable modem termination system (CMTS) is performed. Many of these prior art approaches seek to perform and provide broadband network access to a number of CM users.
CM communication systems are realized when a cable company offers network access, typically Internet, access over the cable. This way, the Internet information can use the same cables because the CM communication system puts downstream data, sent from the Internet to an individual computer having CM functionality, into a communication channel having a 6 MHz capacity. The reverse transmission is typically referred to as upstream data, information sent from an individual back to the Internet, and this typically requires even less of the cable's bandwidth. Some estimates say only 2 MHz are required for the upstream data transmission, since the assumption is that most people download far more information than they upload.
Putting both upstream and downstream data on the cable television system requires two types of equipment: a cable modem on the customer end and the CMTS at the cable provider's end. Between these two types of equipment, all the computer networking, security and management of Internet access over cable television is put into place. This intervening region may be referred to as a CM network segment, and a variety of problems can occur to signals sent across this CM network segment.
One particular deficiency that may arise within the CM network segment is the undesirable introduction of a CFO in the expected clock frequency sent from the CMs within the CM communication system to the CMTS. There do exist some approaches in the prior art to try to estimate this CFO, but these prior art approaches typically fail to provide an efficient solution. As in the general prior art application of trying to perform CFO estimation, these prior art methods and approaches typically involve a high degree of computational complexity. In addition, these prior art methods and approaches often fail to provide for a high degree of accuracy. In order to correct for CFOs, an accurate estimate of the CFO is needed. In packet based communication systems, a known preamble sequence is transmitted with each data packet. The received sequence and the known preamble are usually used at the receiver side to estimate the carrier frequency offset. However, in many cases, the preamble might not be long enough to obtain the needed accuracy for the carrier frequency offset estimate.
In Time Division Multiple Access (TDMA) systems, this does not cause a significant problem as the residual CFO after preamble estimation can be viewed as a time-variant phase offset that can be tracked using data decisions. Moreover, in TDMA systems, the phase variations over the preamble duration is relatively negligible, thus a very accurate initial phase estimate can be obtained from the preamble. In CDMA systems, however, the known preamble spans a relatively longer period than that of a corresponding TDMA system due to data spreading and possible preamble interleaving. In this case, any residual CFO can cause a relatively high phase variation, which prohibits accurate initial phase estimation. Moreover, since in CDMA systems data and preamble are transmitted at the same time, data symbols will suffer the same large phase variation as the preamble. In addition to that, any possible residual CFO can result in loss of orthogonality in the CDMA codes, which can cause significant inter-code-interference (ICI). CDMA based communication systems need a much higher accuracy of carrier frequency estimation than TDMA systems. In many cases, the preamble might not be long enough to provide such accuracy.
Further limitations and disadvantages of conventional and traditional systems will become apparent to one of skill in the art through comparison of such systems with the invention as set forth in the remainder of the present application with reference to the drawings